FCC has been, and will remain for quite some time, the primary conversion process in oil refining. In a typical present-day FCC process, a liquid feed mixture is atomized through a nozzle to form small droplets at the bottom of a riser. The droplets contact hot regenerated catalyst and are vaporized and cracked to lighter products and coke. The vaporized products rise through the riser. The catalyst is separated out from the hydrocarbon stream through cyclones. Once separated, the catalyst is stripped in a steam stripper of adsorbed hydrocarbons and then fed to a regenerator where coke is burned off. The products are sent to a fractionator for fractionation into several products. The catalyst, once regenerated, is then fed back into the riser. The riser-regenerator assembly is heat balanced in that heat generated by the coke burn is used for feed vaporization and cracking. The most common FCC feeds by far are gas oils or vacuum gas oils (VGO) which are hydrocarbon mixtures boiling above about 650.degree. F. When refiners need to convert heavy, or highly contaminated oils such as resids, they usually blend a small amount of such heavy oils with the gas oil feeds. Due to a dwindling supply of high-quality crudes, the trend in the petroleum industry is that FCC will have to convert more and more heavy, dirty feeds. Such feeds contain a high level of contaminants such as nitrogen, sulfur, metals, polynuclear aromatics, and Conradson Carbon Residue (CCR, a measure of asphaltene content). Hereafter, the term heavy component is used to include such highly contaminated hydrocarbons as resids, deasphalted oils, lube extracts, tar sands, coal liquids, and the like. Such heavy components are added to other feeds containing less heavy components to obtain an FCC feed. These heavy components will become a significant portion of FCC feeds in years to come.
The technical problems encountered with FCC feeds containing heavy components have been reviewed by Otterstedt et al., (Otterstedt, J. E., Gevert, S. B., Jaras, S. G., and Menon, P. G., Applied Catalysis, 22, 159, 1986). Chief among them are high coke and gas yields, catalyst deactivation, and SO.sub.x in flue gas. The coke forming tendency of such heavy component-containing feeds has traditionally been gauged by their CCR content. VGO feeds typically contain less than 0.5 wt % CCR, whereas atmospheric and vacuum resids typically contain 1 to 15 wt % and 4 to 25 wt % CCR, respectively. Since cracking of such heavy components can produce coke levels far higher than that required or tolerable by existing FCC units, the maximum permissible level of the heavy component in the FCC feed is often limited by the unit's coke burning capacity. Many FCC units today are capable of cracking only 5-15 wt % resid, or heavy component, in the feed. Due to feed cost considerations, there is a strong need for economical methods that can expand the FCC's operating envelope to enable increased amounts of the heavy component to be utilized in the feeds processed in existing FCC units.
A significant fraction of the cracking and catalyst coking in FCC takes place at the riser bottom where the feed is injected through multiple nozzles. Today's FCC feed injectors typically consist of rings around the riser wall with 6 to 10 nozzles. These nozzles can be at the same elevation or in two rows one above the other. When the FCC feed contains a heavy component such as resid, the standard practice has been to premix the heavy component with gas oil and inject the resulting mixture through all of the nozzles. A major effort in FCC has been directed toward the improvement of the spray pattern to minimize the variation in the catalyst-to-oil ratio over the riser cross-section. For this reason, feed nozzles that produce a flat fan of liquid are gaining wide acceptance these days (see R. J. Glendining, T. Y. Chan, and C. D. Fochtman, NPRA Paper AM-96-25, San Antonio, Tex., Mar. 17, 1996).
Much effort has also been expended on the improvement of cracking selectivity through feed separation. For instance, U.S. Pat. No. 3,424,672 increased gasoline yield by cracking topped crude and low octane light reformed gasoline in separate risers. U.S. Pat. No. 3,617,496 improved gasoline selectivity by fractionating the FCC feed into a low and high molecular weight fractions and then cracking said fractions in separate riser reactors. In U.S. Pat. No. 3,448,037, a virgin gas oil and a cracked cycle gas oil are individually cracked through separate reaction zones to recover higher gasoline products. U.S. Pat. No. 3,993,556 cracked heavy and light gas oils in separate risers to improve yields of high octane naphtha. To recover high volatility gasoline, high octane blending stock, light olefins for alkylation, U.S. Pat. No. 3,928,172 proposed to crack a gas oil feed and heavy naphtha and/or virgin naphtha fraction in separate cracking zones. U.S. Pat. No. 3,801,493 cracked virgin gas oil, topped crude and the like, and slack wax in separate risers to recover a light cycle gas oil fraction for furnace oil use and a high octane naphtha fraction suitable for use in motor fuel, respectively. U.S. Pat. No. 5,009,769 described cracking naphtha in a first riser and cracking gas oils and residual oils in a second riser. To improve conversion to gasoline and olefins, U.S. Pat. No. 5,565,176 disclosed separate cracking of a paraffin rich fraction and a CCR-rich fraction.
The prior art work was primarily driven by the market demand to produce high octane gasoline. What is needed in the art is a method which allows for increased use of alternative feeds containing, for instance, heavy components and stretches the operating limits of existing FCC units with yield improvements.